The following abbreviations are herewith defined, at least some of which are referred to within the following description of the state-of-the-art and the present invention.    ADC Analog-to-Digital Converter    APON ATM PON    BER Bit Error Rate    BI-PON Bit Interleaved PON    BPON Broadband PON    CDR Clock/Data Recovery    CO Central Office    DAC Digital-to-Analog Converter    DBA Dynamic Bandwidth Allocation    EPON Ethernet PON    FEC Forward Error Correction    GE Gigabit Ethernet    GEM GPON encapsulation method    GPON Gigabit PON    IEEE Institute of Electrical and Electronics Engineers    ITU International Telecommunication Union    LNA Low Noise Amplifier    MAC Media Access Control    NRZ Non Return to Zero    OAM Operations, Administration, Management    ODN Optical Distribution Network    OLT Optical Line Terminal    ONT Optical Network Terminal    ONU Optical Network Unit    PC Personal Computer    PCS Physical Coding Sublayer    PMA Physical Medium Attachment    PMD Physical Medium-Dependent    PON Passive Optical Network    PP Packet Processing    RF Radio Frequency    TDM Time Division Multiplexing    TDMA Time Division Multiple Access    TV Television    VoIP Voice over Internet ProtocolNote that the techniques or schemes described herein as existing or possible are presented as background for the present invention, but no admission is made thereby that these techniques and schemes were heretofore commercialized or known to others besides the inventors.
Residential and enterprise network services including television, telephone, Internet access and a number of other emerging services are delivered to subscribers by means of a communications network. These large networks may be conceptually divided into a core network and an access network or networks. The core networks carry large amounts of digitally-encoded information over high-capacity cables or other transmission media. Access networks reach individual residences or multi-tenant dwelling units and other customers such as institutions or businesses to communicate with the core network through which the services are available.
The access network for residential or business subscribers, for example, is usually referred to as a wireline network, which may consist of optical fiber, copper wires, or coaxial cables. Electronic equipment in the network operator's CO (central office) on one end, and at the customer premises equipment at the other, define the access network and communicate using standard or occasionally proprietary protocols. An exemplary access network is illustrated in FIG. 1.
FIG. 1 is a simplified schematic diagram illustrating selected components of a typical PON 100 according to the existing art. In the CO, the OLT 105 provides a connection to the core network (not shown). OLT 105 also performs several management functions such as routing data traffic toward the proper destination and in the proper format and scheduling PON transmissions in both the downstream and upstream directions.
In this example, an optical splitter 110 receives downstream traffic transmitted by OLT 105, typically over a single optical fiber, and splits the signal for continued transmission along multiple fibers, one corresponding to each ONT served. In FIG. 1, ONTs 115a through 115n are shown, each corresponding to a subscriber network. As implied by the ellipsis, there could be any number of ONTs served by OLT 105 via optical splitter 110, though in a typical implementation there may be 32 or as many as 64. Likewise, there may be numerous optical splitters, each dividing a separately transmitted signal from OLT 105, although for simplicity only optical splitter 110 is shown in FIG. 1. The optical splitter 110 is often located in an outside plant, such as a street cabinet that is not part of the CO. It is preferably located relatively closer to the ONTs it serves to minimize the length of the individual cable runs to each ONT.
The content transmitted by OLT 105 to each ONT is not identical, of course, so a TDM scheme is used in the downstream direction, with data addressed to each individual ONT, or in some cases groups of ONTs, sent at a different time. As should be apparent from FIG. 1, however, each ONT 115a through 115n receives the same transmission, but is configured to select its own data based on the destination address included in the transmitted packets.
In the upstream direction, data from each ONT is sent according to a time schedule set by the OLT 105 according to a TDMA transmission scheme. Upstream transmissions are typically carried by a different wavelength of light so as not to interfere with the downstream transmissions as the same optical fibers are used for traffic in both directions. The upstream transmissions from each ONT 115a through 115n arrive at optical splitter 110, which in this direction acts as a combiner for directing the transmitted signal on the fiber returning to the OLT 105.
As might be expected, the communications are usually formatted according to a standard protocol, for example, ITU-T G.984 (GPON) or IEEE 802.3ah (EPON). Most PONs use non-modulated, baseband NRZ signaling for transmissions over the optical fiber.
The ONTs 115a though 115n process the received downstream data traffic and provide to their respective subscriber networks the data from the OLT 105 that is intended for them. They also receive subscriber-network upstream traffic and transmit to the OLT 105 during an assigned time period. The ONT is a demarcation point between the PON and the subscriber network. An exemplary subscriber network is shown in FIG. 2. FIG. 2 is a simplified schematic diagram illustrating selected components of a typical subscriber network 130 according to the existing art.
In network 130 of FIG. 2, ONT 115a is connected to a media gateway 135, which in turn communicates with a number of subscriber devices. These connections are frequently though not universally implemented using coaxial or Cat 5 cables, or by simple copper wires. The type and number of these devices may vary widely, but shown here are a VoIP (voice over Internet Protocol) telephone 140 and a PC (personal computer) 155 such as are currently found in many homes. Also present is a wireless router 160 for communicating over radio channels according to a short-range protocol such as Wi-Fi with, in this example, a laptop 165 and an electronic tablet device 170. A set-top box 145 communicates with media gateway 135 to receive video programming for display on TV (television) 150. Set-top box 145 may perform channel-changing functions as well as video decompression and recording.
The media gateway 135 shown in FIG. 2 is capable of performing a variety of functions including routing, packet switching, firewall protection, and signal processing for VoIP telephone 140. Of course, not all of the devices shown in FIG. 2 are used by all subscribers, and so the functions actually performed by the media gateway will vary from location to location. On the other hand, the use of a single access network for providing a variety of services is growing in popularity. Other devices not shown in FIG. 2 may also be present in subscriber network 130, and its composition is not necessarily static; devices may be added and subtracted on a regular basis.
As mentioned above, the ONT 115a serves as a demarcation point between the PON 100 shown in FIG. 1 and the subscriber network 130 shown in FIG. 2. In this capacity, the ONT is also used for discovery and ranging procedures. Using an agreed protocol, the OLT and ONTs exchange messages so that the OLT can discover which ONTs are operational and connected to the PON, and also their respective distances from the OLT in terms of transmission time. This information is used, for example, for properly allocating bandwidth for upstream transmissions.
The PON is referred to as “passive” because there are no network elements between the OLT and the CPE that consume power; the optical splitter or splitters that divide the downstream traffic and combine the upstream traffic are passive devices. The energy consumed by the ONT and other subscriber equipment at the CPE end is substantial, however, and it is increasing as advancements in PON technology have increased the maximum bit rate. In some cases, the increase in energy consumption proportionally exceeds the increase in capacity. Some solutions have been proposed, such as reducing ONT functionality during battery-powered operation or going to a reduced-power state during scheduled intervals without data traffic.
Needed, however, is a way to reduce power consumption during such times, and in periods of normal operation as well. This need and other needs relating to PON networks are answered by the present invention.